People with normal hearing are able to localize sound (i.e., locate the direction of the source of the sound) by detecting various binaural cues with both ears. For example, sound originating from the right side of a person reaches the person's right before it reaches the person's left ear. The sound also has a higher level (i.e., amplitude) at the person's right ear than the person's left ear because the person's head shadows the left ear. These time and level differences as detected by the ears allow the person's auditory system to determine that the sound is coming from the person's right side.
Unfortunately, because sound localization requires the use of both ears, a unilateral cochlear implant patient (i.e., a person fitted with a cochlear implant in only one ear) is often incapable of localizing sound (assuming that the patient cannot hear with the non-implanted ear). Bilateral cochlear implant systems (i.e., systems in which a patient is fitted with a cochlear implant in both ears) have provided some degree of sound localization for cochlear implant patients. However, conventional cochlear implant systems have difficulty accurately representing binaural cues. Hence, the sound localization capabilities of bilateral cochlear implant patients are often quite poor.